The present invention relates to methods and compositions for treating subterranean formations, and more specifically, to improved methods and compositions for reducing the viscosity of viscosified treatment fluids.
Viscosified treatment fluids are used in a variety of operations in subterranean formations. For example, viscosified treatment fluids have been used as drilling fluids, fracturing fluids, diverting fluids, and gravel packing fluids. Viscosified treatment fluids generally have a viscosity that is sufficiently high to suspend particulates for a desired period of time, to transfer hydraulic pressure to divert treatment fluids to another part of a formation, and/or to prevent undesired leak-off of fluids into a formation from the buildup of filter cakes.
Most viscosified treatment fluids include gelling agents that may increase a treatment fluid's viscosity. The gelling agents typically used in viscosified treatment fluids usually comprise biopolymers or synthetic polymers. Common gelling agents include, inter alia, galactomannan gums, such as guar and locust bean gum, cellulosic polymers, and other polysaccharides.
In some applications, e.g., in subterranean well operations, after a viscosified treatment fluid has performed its desired function, the fluid may be “broken,” wherein its viscosity is reduced. Breaking a viscosified treatment fluid may make it easier to remove the viscosified treatment fluid from the subterranean formation, a step that generally is completed before the well is returned to production. Breaking of viscosified treatment fluids is usually accomplished by incorporating “breakers” into the viscosified treatment fluids. Traditional breakers include, inter alia, enzymes, oxidizers, and acids. As an aside, a viscosified treatment fluid may break naturally if given enough time and/or exposure to a sufficient temperature. Such an approach is generally not practical though as it may increase the amount of time before the well may be returned to production.
Oxidizing breakers, such as peroxides, persulfates, perborates, oxyacids of halogens and oxyanions of halogens, are typically used to break viscosified treatment fluids at temperatures above 200° F., e.g., by oxidative depolymerization of the polymer backbone. However, in lower temperature regimes these oxidizing agents may be ineffective for breaking the viscosity within a desirable time period. For example, when using a chlorous acid oxidizing breaker below about 200° F., a breaker activator is required to break the polymer in a desirable time period. Previous solutions have used a cupric ion chelated with ethylenediaminetetraacetic acid (EDTA) or iron citrate to activate the breaker; however, these compounds can have numerous disadvantages. For example, EDTA may be associated with potential detrimental effects on ocean species. Additionally, citrate compounds may have less desirable solubility characteristics. Also, the iron and citrate may be weakly chelated, which can allow the iron to precipitate into the environment.